Notes on R for Data Science Chapter 5: Data transformation

Ideas and exercises come from https://r4ds.had.co.nz/transform.html

Additional notes by TCS

Setup

First, we load the tidyverse package and a dataset. This data frame contains all 336,776 flights that departed from New York City in 2013.

require(nycflights13)

Loading required package: nycflights13

require(tidyverse)

Loading required package: tidyverse
Registered S3 methods overwritten by 'dbplyr':
  method         from
  print.tbl_lazy     
  print.tbl_sql      
── Attaching packages ──────── tidyverse 1.3.2 ──✔ ggplot2 3.3.6      ✔ purrr   0.3.4 
✔ tibble  3.1.8      ✔ dplyr   1.0.10
✔ tidyr   1.2.1      ✔ stringr 1.4.1 
✔ readr   2.1.2      ✔ forcats 0.5.2 ── Conflicts ─────────── tidyverse_conflicts() ──
✖ dplyr::filter() masks stats::filter()
✖ dplyr::lag()    masks stats::lag()

flights

Basic dplyr functions

There are 5 key functions (“verbs”), plus a helper function, that do most data manipulation tasks in dplyr:

• filter: pick observations by values
• arrange: reorder rows
  
• select: pick variables by name
  
• mutate: create new variables from existing ones, using functions
  
• summarise: collapse values into single ones
  
• group_by: change scope of a verb from the whole dataset to individual groups

How verbs work

Input and output are data frames. The input is never modified.

Arguments consist of
1. A data frame
2. “What to do with the data frame”

filter() gives you a subset of rows based on values

Available comparison operators are >, >=, <, <=, != (not equal), and == (equal).

Wrapping the assignment in parentheses also prints out a preview of the resulting dataframe.

jan1 <- filter(flights, month == 1, day == 1)
jan1

(dec25 <- filter(flights, month == 12, day == 25))

For floating-point numbers, instead of relying on ==, use near() to avoid unwanted inequality due to rounding:

sqrt(2) ^ 2 == 2

[1] FALSE

1 / 49 * 49 == 1

[1] FALSE

near(sqrt(2) ^ 2,  2)

[1] TRUE

near(1 / 49 * 49, 1)

[1] TRUE

You can also use:
* logical operators &, |, !, xor
* %in% constructions

(nov_dec <- filter(flights, month == 11 | month == 12))

(jan_mar <- filter(flights, month %in% seq(1,3)))

NA handling in filter

filter() includes ONLY rows where the condition is TRUE; it excludes both FALSE and NA values. If you want to preserve missing values, ask for them explicitly:

df <- tibble(x = c(1, NA, 3))
(biggerthan1 <- filter(df, x > 1))

(bigorNA <- filter(df, is.na(x) | x > 1))

Exercises for filter()

1. Find all flights that
   

• Had an arrival delay of two or more hours
• Flew to Houston (IAH or HOU)
  
• Were operated by United, American, or Delta
  
• Departed in summer (July, August, and September)
  
• Arrived more than two hours late, but didn’t leave late
  
• Were delayed by at least an hour, but made up over 30 minutes in flight
  
• Departed between midnight and 6am (inclusive)
  

2. Another useful dplyr filtering helper is between(). What does it do? Can you use it to simplify the code needed to answer the previous challenges?
   
3. How many flights have a missing dep_time? What other variables are missing? What might these rows represent?
   
4. Why is NA ^ 0 not missing? Why is NA | TRUE not missing? Why is FALSE & NA not missing? Can you figure out the general rule? (NA * 0 is a tricky counterexample!)

# from ?flights we learn that the delay columns are in minutes
(delay2hr <- filter(flights, arr_delay>=120))

(intohouston <- filter(flights, dest == "IAH" | dest == "HOU"))

(someairlines <- filter(flights, carrier %in% c("UA", "AA", "DL")))

(summer <- filter(flights, month %in% c(7,8,9)))

(gotlate <- filter(flights, dep_delay <= 0 & arr_delay >= 120))

(madeup <- filter(flights, dep_delay >= 60 & (dep_delay-arr_delay > 30)))

(redeye <- filter(flights, dep_time < 600 | dep_time == 2400)) #2400 is midnight

(betweensummer <- filter(flights, between(month, 7, 9))) # inclusive

(missingdeptime <- filter(flights, is.na(dep_time) == TRUE)) # all missing arrival times and some missing tail numbers. probably cancelled flights

(NA^0)

[1] 1

(NA|TRUE)

[1] TRUE

(FALSE & NA)

[1] FALSE

(NA * 0)

[1] NA

(Inf * 0)

[1] NaN

(Inf ^ 1)

[1] Inf

Notes on NAs

A missing value can look like a real one. Certain operations always give a numerical or logical result:
* NA ^ 0 = 1
* NA|TRUE = TRUE because one of the arguments is true
* FALSE & NA = FALSE because one of the arguments is false

NA * 0 is “a tricky counterexample”. You would think that anything times 0 is 0. However, it could be infinity (Inf) which would be undefined. Hence the expression cannot be evaluated. (Unlike Inf^0 which still = 1.)

arrange() is for sorting rows

• The arguments for arrange() are a dataframe and column name(s).
  
• The desc option reverses the order.
  
• Missing values (NAs) are always at the end regardless of desc()

Remember that rows are in no particular order even if you’ve appended them in some order!

(bydate <- arrange(flights, year, month, day))

(longestfirst <- arrange(flights, desc(dep_delay)))

df <- tibble(x = c(5, 2, NA))
(mytib <- arrange(df, x))

(mytibdesc <- arrange(df, desc(x)))

Exercises for arrange()

1. How could you use arrange() to sort all missing values to the start? (Hint: use is.na()).

Note: FALSE (0) sorts before TRUE (1) so we need to use !is.na()

2. Sort flights to find the most delayed flights. Find the flights that left earliest.
   
3. Sort flights to find the fastest (highest speed) flights.
   
4. Which flights travelled the farthest? Which travelled the shortest?

(nafirst <- arrange(flights, !is.na(dep_time))) # FALSE comes before TRUE

(earliestdelayed <- arrange(flights, desc(dep_delay), dep_time))

(fastest <- arrange(flights, desc(distance/air_time)))

(farthest <- arrange(flights, desc(distance)))

(shortestflights <- arrange(flights, distance))

select() gives you a subset of columns by name

You can name each column, or specify a range using a colon.
As with other R selections, you can omit certain columns using the minus sign.

(datedata <- select(flights, year, month, day))

(bunchocols <- select(flights, year:day))

(nodates <- select(flights, -(year:day)))

Partial names

select() does not have to use exact column matches.

You can use partial names and regular expressions:

• starts_with("foo")
  
• ends_with("bar")
  
• contains("foobar")
  
• matches(some_regex)
  
• num_range("x", 1:3) matches x1, x2 and x3

Variants on select()

You can use select() to rename and re-organize columns to some extent. For example:

• rename() is considered a variant of select() where you take a column, change its name, and keep all other columns as well. If you use select() to rename a column you will lose all other columns.
  
• everything() is a helper for select() that lets you move one or a few columns to the beginning (left) of the table, while retaining all other columns.

Exercises for select()

1. Brainstorm as many ways as possible to select dep_time, dep_delay, arr_time, and arr_delay from flights.

2. What happens if you include the name of a variable multiple times in a select() call?

3. What does the any_of() function do? Why might it be helpful in conjunction with this vector?

vars <- c("year", "month", "day", "dep_delay", "arr_delay")

4. Does the result of running the following code surprise you? How do the select helpers deal with case by default? How can you change that default?

select(flights, contains("TIME"))